The construction universally used for commercial aircraft is that of a skeleton framework comprising a plurality of spaced parallel transverse frames connected by longitudinally extending stringers, the framework being covered externally by a thin stressed metal skin, and internally by decorative removable interior trim panels. Each cell between each pair of immediately adjacent frames and immediately adjacent stringers is provided with its own insulating element, or a stack of elements to obtain the required thickness, a vapor barrier being provided on the warm side. In aircraft systems the vapor barrier usually is provided by enclosing each element completely in a bag of a thin flexible moisture-impervious plastics material to form what is commonly referred to in the industry as a pillow or blanket.
These aircraft insulation systems are subjected to conditions that are not encountered by those used, for example, in land-based installations such as buildings and automobiles. For example, the range of atmospheric pressure to which the system is subjected, and the rate at which that pressure changes, are much greater than the usual diurnal range due to changes in weather conditions, even though in the interest of passenger comfort the pressure inside the aircraft is maintained within a restricted range. With a land-based system the change of pressure caused by weather changes is usually in the range from 99 kPa to 104 kPa, and although changes of from 97 kPa to 106 kPa theoretically are possible they are unusual. Moreover, even with an abrupt weather change the rate of change of the pressure will still be much less than with an aircraft. Any increase in altitude is accompanied by a decrease in ambient pressure, but again in the case of, for example an automobile climbing into hilly country, the rate of change is much slower than with an aircraft. High flying aircraft have a substantially airtight fuselage, minor leakage outlets being present such as the toilet discharges, and the interior pressure is controlled to be as low as possible without passenger discomfort. Upon takeoff the cabin pressure is allowed to drop to a value within the range equivalent to that outside at about 2,400-3,600 meters (8,000-12,000 feet), all and any changes in the cabin pressure change being transmitted throughout the fuselage interior through the joints in the floor and between the cabin ceiling trim panels.
Another important difference in the conditions encountered is the rate of change of the ambient temperature to which an aircraft is subjected. The range of ambient temperature for which an aircraft must be designed is about the same as for other types of vehicles, in that both may be employed in hot (e.g. equatorial) locations where the vehicle surface temperature in direct sunlight can reach about 50.degree. C. (132.degree. F.), or in cold (e.g. arctic/antarctic) locations where the outside temperature can be as low as -50.degree. C. (-60.degree. F.). However, with a building or a vehicle other than an aircraft the normal diurnal temperature variation to which it is subjected usually is much smaller than this maximum range, in that hot location temperatures may rarely if ever go below zero, and cold location temperatures may rarely if ever go above zero, while the rate of change is also low. An aircraft taking off from, or landing at, an equatorially located airport can be subjected to almost the full range at a high rate of change, the period being only the relatively short time required to reach cruising altitude, or to descend and land. The frequency of change is also very much greater, in that even a long haul aircraft will usually make a number of ascents and descents during a working day, while short haul aircraft such as are used for hub connecting traffic will make a much larger number of ascents and descents in that period.
The atmospheric temperature is almost always below zero celsius above about 2,400 meters (8,000 feet), and most passenger flights are well above that altitude, while for passenger comfort the temperature inside the cabin must of course be kept within a much more restricted range of about 20.degree. C.-24.degree. C. (68.degree. F.-75.degree. F.). It is inevitable therefore that comparatively frequently the temperature inside each insulation element adjacent to the outer metal skin will reach the dew point at which any water vapor inside the element bag will condense to water, while the temperature closer to the outer metal skin will be sufficiently low for it to deposit as ice.
The insulating materials used for the elements are primarily intended to meet relatively rigorous requirements as to noise, vibration and fireproofing, and thermal insulation although important is secondary to these; as is desirable with aircraft they should also be of the lowest possible weight. The material currently most commonly used is a special lightweight glass fibre, although recently this has begun to be replaced, at least partly, by specially developed polyimide open cell foams, which however are considerably more expensive than the glass fibre materials. All of these materials essentially are porous and filled with air. The air pressure within the elements must be allowed to equalize with the cabin pressure, since with an impervious vapor barrier either the elements will bulge into the cabin as the cabin pressure drops, possibly dislodging the trim panels, or they will collapse inward as it increases and considerably reduce their insulation efficiency. To prevent these undesirable effects the envelopes are usually provided with relatively large openings, and it has been accepted that there will a resulting exchange of air between the cabin interior and the interiors of the elements. Even a small opening is highly effective in allowing the passage of water vapor from the more humid cabin interior into the less humid element interiors under the effect of its partial vapor pressure, which causes its rapid dispersion throughout the element interior. The transfer is made even more effective by lower temperatures in the element interiors, resulting in lower effective partial vapor pressures, so that more vapor is required for the equalisation. The transfer increases even further when any part of the element interior is below the dew point temperature, in that the water vapor condenses, further reducing the partial vapor pressure, so that even further transfer takes place.
This undesirable effect is exacerbated with high flying passenger aircraft during the periods when the relatively tightly closed aircraft cabin contains the crew and a large number of passengers, each of which adds moistened air to the cabin atmosphere via beverages, breathing and perspiration, so that the humidity is increased. As a result, during the period that the aircraft is at normal flying altitude, there can be considerable condensation of moisture within the insulation, reducing its effectiveness as acoustic insulation, so that the interior noise increases, and also reducing its effectiveness as thermal insulation, so that fuel consumption is increased. Such condensed moisture is found to have a particularly deleterious effect upon glass fibre insulation in that it already has a tendency to shake downward toward the bottom of its bag, and the weight of the moisture increases this tendency. When the amount of condensate becomes greater than the surface retentive capacity of the insulation material it pools in the bottoms of the cells, from which it usually is arranged to drain into a space between the elements and the metal outer skin, from whence it drains to the aircraft bilge for eventual discharge. The condensate adds undesired weight to the aircraft, and it has been estimated that a 300 seat aircraft can accumulate as much as 1,000 Kg (2,000 pounds) of excess weight in the form of such condensate, correspondingly increasing fuel consumption, and surreptitiously and dangerously increasing take-off weight. A not inconsiderable disadvantage is that occasionally, especially as the aircraft accelerates or decelerates, pooled condensate in the cells above the cabin ceiling leaks on to the passengers below.